PAC-activated sludge

Bornhardt, C., J.E. Drewes, and M. Jekel. 1997. Removal of organic halogens (AOX) from municipal wastewater by powdered activated carbon (PAC)/activated sludge (AS) treatment. Water Sci. Technol. 35 147-153. 

Recently developed applications of activated carbon to wastewater treatment involve the addition of PAC to conventional activated sludge aeration tanks. The combination of PAC with biological process is often times referred to as the PACT or PAC-activated sludge process (3). The PACT process has attracted a great deal of interest because it is a method by which the performance of a waste treatment facility may be improved in various areas. Technical advantages that can potentially be achieved by adding PAC to biological reactors include ... 

The PAC-activated sludge process is shown in Fig. 1. The activated sludge process is used extensively to degrade municipal as well as industrial wastes. Because activated... 

The required design considerations for PAC-activated sludge wastewater treatment systems are ... 

Carbon type and dosage are crucial variables that affect the performance of the PAC-activated sludge process. Both variables are significant in predicting effluent dissolved organic carbon (DOC) as shown in Fig. 7 (15). While the shape of these curves is generally similar for different wastes and carbon types, the coordinates vary with each wastewater and should be determined in the laboratory. 

Another study examined the PAC-activated sludge process (14). The addition of powdered carbon directly into mixed liquor in an activated sludge plant aeration basin is referred to as the PACT process. The benefits attributed to this system are numerous, such as improved BOD and COD removal by adsorption, adsorption of color and toxic agents, reduction of aerator and effluent foam, and so on. The goal of the PACT process is to improve organic removal or to improve sludge settling characteristics. 

In the United States, PAC-activated sludge systems for nitrification generally has been applied at municipal treatment plants where industrial sources contribute a significant fraction of the incoming wastewater. In all instances, PAC regeneration was included in the flowsheet (38). A summary of selected municipal PAC facilities is presented in Table 8. 

The procedure to foUow in designing PAC-activated sludge systems for nitrification involves a modification to those for complete mix or conventional plug flow systems (39) in order to account for the effects of the addition of PAC. According to major suppliers of the technology (38 2), most PAC process systems are designed at MUSS concentrations of approx 15,000 mg/L. The mixed liquor is composed of volatile activated carbon, biomass, nonvolatile PAC ash, biomass decay components, and influent inert material. The... 

Most carbon adsorption units use granular activated carbon (GAC). The powdered form of activated carbon (PAC) typically is less than 100 microns in diameter and may be used to reduce dioxins in incinerator emissions (2) and in the treatment of drinking water and wastewater treatment (see the section on "Activated Sludge"). 

The addition of powdered-activated carbon (PAC) to the activated sludge process has received considerable attention, particularly with respect to the removal of specific organics. The applicability of activated carbon in removing specific substrates depends on the molecular weight, solubility, polarity, location of functional groups, and overall molecular configuration. Investigations of PAC systems have centered around process enhancement factors. These include ... 

Addition of PAC to the activated sludge process increases the soluble chemical oxygen demand (SCOD) removal from the pharmaceutical wastewater but no measurable effect in terms... 

ASP, activated sludge process PAC-ASP, powdered activated carbon activated sludge process GAC, granular activated carbon TOC, total organic carbon. 

Addition of PAC in the activated sludge process was found beneficial in improving the effluent quality, but it cannot be recommended until the problem of viscous layer formation is solved. 

A powdered activated carbon fed activated sludge process is designed to treat 15,000 GPD of pharmaceutical wastewater. The SCOD (soluble chemical oxygen demand) of the treated effluent is 590 mg/L. Determine the dose of PAC (powdered activated carbon) required for further reduction of effluent SCOD from 590 mg/L to 200 mg/L. Use the Freundlich equation [48] XjM = (3.7 x 10 ) to determine the dose of powdered activated carbon. 

The result of a pilot plant study of PAC-fed activated sludge process is given in the following table. 

The PACT process consists of the addition of PAC to the aeration basin in an activated sludge plant as shown in Figure 2. Depending on carbon usage, regeneration of the carbon may be required to optimize the economics of the process. At present, regeneration using wet air oxidation(WAO) would appear to be the mo cost-effective approach. 

PACT is registered trade name for an integrated activated sludge/powdered activated carbon (AS/PAC) process patented by E. I. DuPont, Inc. (U.S. Patent 3,904,518) and marketed by Zimpro, Inc. 

The PAC activated slndge system is a modified activated sludge process. PAC is added to the aeration tank where it is mixed with the biological solids. The mixed liquor solids are settled and separated from the treated effluent in a gravity clarifier. Polyelectrolyte will normally be added prior to the clarification step to enhance solids-liquid separation. If phosphorus removal is necessary, alum is often added at this point. Even with polyelectrolyte addition, tertiary filtration is normally required to reduce the level of effluent suspended solids. The clarifier nnderflow solids are continuously returned to the aeration tank. A portion of the carbon-biomass mixture is wasted periodically to maintain the desired solids inventory in the system. 

When PAC is dosed into an activated sludge process for combined adsorption and biochemical reactions, the combined process is also called the PACT process, in which PAC still stands for powdered activated carbon, while ACT stands for activated sludge. 

This PAC contains 1—2% sulfate as soluble calcium sulfate. Sulfate has been found to make PAC products unstable precipitate forms in less than one week at 50°C. Sulfate, however, has also been seen to increase PAC activity in water clarification and is thus intentionally added in one preparation (24). Precipitated calcium sulfate creates a sludge disposal problem. Typical Al content as AI2O2 of PAC products made from alum is 6 —8%. 

Treatment of waste waters can be also carried out using powdered carbons but only the treatment with PAC can be used independently only in specific cases. In the most common technology, PAC is used together with activated sludge [101]. The powdered activated carbon... 

The presence of PAC improves sludge sedimentation. The more activated carbon that is added, the more sludge is produced. Care should be taken regarding dust formation. A disadvantage is the color (black sedimentation). The presence of activated carbon can influence the disposal possibilities for the excess sludge. 

In the second instance, according to some practice in the U.S. petrochemical industry, powdered activated carbon (PAC) is added directly into the aeration tank in amounts that may vary from 10 to 50 mg l . At the same time, however, advanced activated sludge age (40 to 45 days) is maintained and considerable reductions (from 10 to 20%) in TOC have been achieved. 

Fig. 1. Alternative wastewater treatment technologies, where GAC = granular activated carbon, PAC = powdered activated carbon, POTW = publicly owned treatment works, and RBC = rotating biological contactor (— ), wastewater return flows (—— ), sludge.

Acbvated carbon adsorption is most often employed for removal of organic constituents from water and wastewater. Granular acbvated carbon (GAC) or powdered activated carbon (PAC) may be used. Granular activated carbon columns can be used for secondary tteatment of industtial wastewaters or for tertiary tteatment to remove residual organics from biological tteatment effluent. The primary use of PAC in wastewater tteatment has been in the PACT process (Zimpro), in which PAC is added to the acbvated sludge process for enhanced performance. This process is discussed in the next section of this chapter. 

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